fix: clean also requires ARCH

makefile

@@ -45,6 +45,7 @@ define wr_colour
 endef
 
 # set the correct environment variables depending on the platform
+ifneq ($(MAKECMDGOALS),clean)
 ifeq ($(ARCH),linux-x86_64)
 CFLAGS += -target x86_64-pc-linux-gnu
 else ifeq ($(ARCH),win-x86_64)
@@ -53,6 +54,7 @@ EXT=.exe
 else
 $(error you must set the ARCH environment variable to one of these: 'linux-x86_64' 'win-x86_64')
 endif
+endif
 
 # compiles and execute the binary
 run: compile